I agree with db, they are very nice for trolling and will save you money on snagged lures. Two years ago I left mine home and was frustrated as I trolled and kept snagging in the rocks.  Waste valuable time going back and trying to free the lure.  I also lost two $10 rapala divers and now will never leave the depth finder behind.

If you don't troll much they are invaluable for finding reefs and structure.  It is just a time saver as you don't spend time in unproductive structure ie: a flat featureless bottom.